Abstract
Abstract. Mesoscale eddies play a major role in controlling ocean biogeochemistry. By impacting nutrient availability and water column ventilation, they are of critical importance for oceanic primary production. In the eastern tropical South Pacific Ocean off Peru, where a large and persistent oxygen-deficient zone is present, mesoscale processes have been reported to occur frequently. However, investigations into their biological activity are mostly based on model simulations, and direct measurements of carbon and dinitrogen (N2) fixation are scarce.We examined an open-ocean cyclonic eddy and two anticyclonic mode water eddies: a coastal one and an open-ocean one in the waters off Peru along a section at 16° S in austral summer 2012. Molecular data and bioassay incubations point towards a difference between the active diazotrophic communities present in the cyclonic eddy and the anticyclonic mode water eddies.In the cyclonic eddy, highest rates of N2 fixation were measured in surface waters but no N2 fixation signal was detected at intermediate water depths. In contrast, both anticyclonic mode water eddies showed pronounced maxima in N2 fixation below the euphotic zone as evidenced by rate measurements and geochemical data. N2 fixation and carbon (C) fixation were higher in the young coastal mode water eddy compared to the older offshore mode water eddy. A co-occurrence between N2 fixation and biogenic N2, an indicator for N loss, indicated a link between N loss and N2 fixation in the mode water eddies, which was not observed for the cyclonic eddy. The comparison of two consecutive surveys of the coastal mode water eddy in November 2012 and December 2012 also revealed a reduction in N2 and C fixation at intermediate depths along with a reduction in chlorophyll by half, mirroring an aging effect in this eddy. Our data indicate an important role for anticyclonic mode water eddies in stimulating N2 fixation and thus supplying N offshore.
Highlights
Reactive nitrogen (N) limits primary production in large parts of the ocean (Codispoti, 1989)
While PO34− concentrations were not considerably different in the eddy compared to surrounding waters, NO−3 concentrations showed a pronounced decrease in the oxygen-deficient zones (ODZs) of eddy centers at ∼ 76◦ W (eddy A) compared to surrounding waters
Our results demonstrated enhanced N2 fixation rates connected to two anticyclonic mode water eddies off Peru, while elevated N2 fixation was not observed in the cyclonic eddy
Summary
Reactive nitrogen (N) limits primary production in large parts of the ocean (Codispoti, 1989). Some recent modeling studies have suggested a close spatial link between fixed N loss, i.e., N2 production via anammox and/or denitrification, occurring in oxygen-deficient zones (ODZs), and N2 fixation taking place in the adjacent surface ocean with the consequence that the potential habitat of N2fixing organisms is larger than previously thought (Deutsch et al, 2007). As both processes are favored under oxygen-depleted conditions and as some organisms responsible for these processes do not need light, their coupling in ODZ waters would damp excursions in the oceanic N inventory and promote the stability of the global N budget. The biogeochemical significance of noncyanobacterial diazotrophs (i.e., microbes capable of N2 fixation) has been described, and their enormous potential to fix N2 in the ETSP seems to depend on organic-matter supply (Fernandez et al, 2015)
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